Field of the Invention
The invention relates to a device for mounting a substrate to be coated. The device includes a susceptor as a support for the substrate to be coated. The invention further relates to a method for producing an insert for such a susceptor.
The growth of a monocrystalline layer takes place at a much higher temperature in the case of silicon carbide than in the case of other semiconductors such as silicon or gallium arsenide. A primary problem in the field of silicon carbide epitaxy is therefore the choice of a suitable material as support and fixing device of the SiC wafer. The reason for this is, on the one hand, the high process temperature of between 1300xc2x0 C. and 2000xc2x0 C. and, on the other hand, the necessity of maintaining a hydrogen partial pressure in the process chamber of about 10 kPa (0.1 atm), in order to enable monocrystalline growth with acceptable growth rates ( greater than 1 xcexcm/h). Moreover, it is necessary to prevent the material which is in direct contact with the SiC substrate from reacting with the substrate.
On account of these requirements, coated and uncoated graphite and also materials made of transition metals are materials used in the prior art.
Published, Non-Prosecuted German Patent Application DE 38 37 584 A1 discloses a coated substrate mount. In this case, an SiC layer is applied to the surface of a carbon base material.
Uncoated graphite is dimensionally stable and chemically completely inert with respect to SiC. Even with very high temperature gradients, cracking due to thermal stresses does not occur.
At the aforementioned high process temperature of 1300 to 1600xc2x0 C. and a hydrogen partial pressure of about 10 kPa, however, a reaction occurs with the hydrogen to form hydrocarbons. Even if the removal rates are of the order of magnitude of 1 xcexcm/h, these hydrocarbons shift the ratio of carbon to silicon, the ratio being predetermined by the process gases used, such as e.g. silane or propane, in the region of the substrate surface toward an excess of carbon. This effect is location-dependent and produces inhomogeneities in the substrate. Moreover, impurities contained in the graphite are released (Al, Ti, B) and incorporated into the epitaxial layer. The properties of this layer are adversely affected as a consequence.
In order to remedy this, in recent years increasing use has been made of graphite parts for mounting the wafer, that are coated with silicon carbide (SiC). A reaction between the hydrogen and the graphite can be prevented by making this coating gas-tight.
On the other hand, the coating causes the mechanical properties of the parts being adversely affected to a considerable extent. Temperature gradients in the mechanical parts can lead to cracks in the coating and even to a fracture of the entire part. Even non-continuous or non-penetrating cracks lead to an uncontrollable influence on the temperature distribution in the case of the inductive heating which is usually used.
Furthermore, there is the problem of undesired growth of SiC occurring due to the contact between the wafer and a surface, coated with SiC. This problem is intensified by the high process temperatures. The transport process takes place in particular from the support to the rear side of the wafer.
The literature likewise discloses the use of transition metals such as Mo and Ta as susceptor for SiC epitaxy. Both transition metals have a low vapor pressure and are not attacked by the hydrogen. Like graphite, these metals endure high thermal stresses.
However, the diffusion constants of impurities in these metals are high, with the result that e.g. Al contained in the transition metals can, due to this process, pass into the process atmosphere and adversely affect the properties of the epitaxial layer. Moreover, both metals form suicides and carbides, and thus a reaction occurs with the rear side of the wafer and/or with the cleavage products of the silane and propane. As a result, the optical properties of the material surface change in a time-dependent manner. Since a substantial part of the heat transport at the high process temperatures takes place via radiation, this once again results in undesirable perturbations of the temperature distribution of the wafer support. Moreover, Ta tends to include hydrogen in its crystal lattice at high temperatures, which leads to the destruction of the workpiece during a rapid cooling-down process. Therefore, the cooling-down process usually has to be halted for approximately 1 h at about 1000xc2x0 C. and the H atmosphere has to be exchanged e.g. for Ar. This leads to an undesirable lengthening of the whole process and to higher costs.
It is accordingly an object of the invention to provide a device for mounting a substrate which overcomes the above-mentioned disadvantages of the heretofore-known devices of this general type and which rules out a contamination of the substrate during processing, e.g. during production of an epitaxial layer on a wafer. It is a further object of the invention to provide a method for producing an insert for a susceptor.
With the foregoing and other objects in view there is provided, in accordance with the invention, a device for mounting a substrate to be coated, including:
a susceptor for supporting a substrate;
the susceptor including an insert, which has a surface; and
a metal carbide layer of a given thickness forming at least a portion of the surface.
The invention is based on replacing at least parts of the auxiliary materials which are in the high-temperature region during the SiC epitaxy process by metal carbides. To that end, it is proposed to seal the surface of the parts which come into contact with the SiC substrate under the aforementioned ambient conditions, that is to say to coat the surface in a gas-tight manner, with the result that no contaminating material from the parts can be incorporated or diffuse into the substrate via the surface. In this case, high temperature stable carbides should be chosen which are both inert with respect to hydrogen and are in chemical equilibrium with respect to SiC. Furthermore, the carbide-forming agent must not be an electrically active impurity in the SiC. In other words, substances such as Al and boron carbides are therefore ruled out. The following substances are particularly suitable: TaC, MoC, NbC, WC.
The device according to the invention for mounting a substrate, includes a susceptor as a support for the substrate to be coated. The susceptor includes an insert, whose surface is at least partly covered with a metal carbide layer of a predetermined thickness.
The insert is preferably constructed in such a way that it has a xe2x80x9csandwichxe2x80x9d structure in its cross section, that is to say it includes a core, preferably made of graphite or a metal, which is covered with the metal carbide layer.
The thickness of the metal carbide layer on the insert may decrease with an increasing distance from the substrate. The substrate may for example be a wafer. As a result, mainly the surface in direct proximity to the substrate or wafer is sealed, so that there is no diffusion of contaminating material to the wafer from parts which directly come into contact with the wafer. As a result, the production costs of the corresponding parts which come into contact with the wafer are also reduced.
The metal carbide layer is preferably composed of TaC, MoC, NbC or WC.
The insert may include a plurality of tiles each for one substrate, in which case, furthermore, each tile may have a depression for a substrate.
With the objects of the invention in view there is also provided, a method for producing an insert for a susceptor, which includes the steps:
producing a metallic preform;
bedding the metallic preform in a carbon-containing powder;
heating the metallic preform and the carbon-containing powder to an elevated temperature for providing a heat-treated preform;
hard processing the heat-treated preform for providing a hard-processed preform having a surface layer made of a metal carbide; and
disposing the hard-processed preform as an insert on a susceptor.
The method according to the invention for producing inserts with a surface layer made of metal carbides is based on the heat treatment or tempering of a preform embedded in SiC powder between 1500xc2x0 C. and 2000xc2x0 C. and the subsequent hard processing or machining to provide a final shape. This process produces a composite material including a carbide surface layer of greater or lesser thickness, depending on the heat-treatment conditions, and metal that is uncarburized or uncarbided in the interior. The grain boundaries in this metal are likewise at least partly carburized or carbided in this case.
The method for producing an insert for a susceptor with a surface layer made of metal carbide includes the steps of producing a metallic preform, embedding the metallic preform in a carbon-containing powder, heating the metallic preform and the carbon-containing powder to an elevated temperature, hard processing the heat-treated preform and disposing the hard-processed preform on the susceptor as an insert.
The elevated temperature in the heating step is preferably, between 1500xc2x0 C. and 2000xc2x0 C. The heating is preferably performed under elevated pressure.
The carbon-containing powder includes, in particular, silicon carbide powder.
The invention has the following advantages. There is no reaction between the substrate or wafer environment and the SiC and also the epitaxy process gas, such as e.g. propane, silane and hydrogen.
As a result, the purity of the epitaxial layer is improved and a longer service life of the workpiece is achieved. The mechanical properties of the substrate mounting components are good since carbides are intrinsically brittle, but the overall form is stabilized by the ductile metallic core, thus enabling greater flexibility in the configuration of susceptor, sample holder, etc. Moreover, the diffusion in the carbides with respect to the metals is greatly reduced. In particular, the grain boundaries in the metallic microstructure act as xe2x80x9cdiffusion brakesxe2x80x9d. Improved purity of the epitaxial layer is thus achieved. Finally, the inclusion of hydrogen in the crystal lattice is prevented, thereby enabling efficient process control.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a substrate mount for SiC epitaxy and a method for producing an insert for a susceptor, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.